1. Field of the Invention
This invention relates to a reversible thermal recording medium on which an image is recorded or erased by utilizing a reversible change in a recording layer caused by a change in temperature and to a method of producing this medium.
2. Description of the Related Art
Recently, the development of reversible thermal recording mediums capable of recording or erasing an image with heat have been promoted. Reversible thermal recording mediums heretofore known include one based on using a physical change, i.e., an organic low-molecular type (e.g., the one disclosed in Japanese Patent Laid-Open Publication No.S55-154198) and U.S. Pat. No. 4,695,528, issued Sep. 22, 1987 in which transition between a slightly-opaque state and a transparent state can be reversibly repeated with a change in heating temperature and in which one of these states can be maintained with stability at a temperature lower than a certain point, and one based on utilizing a chemical change, i.e., a lueco dyestuff type (e.g., the one disclosed in Japanese Patent Laid-Open Publication No. H2-188294) which consists of a mixture of a lueco dyestuff and a color developing/subtracting agent, in which an organic material having both hydroxyl and carboxyl in a molecule and having a property such as to reversibly release hydrogen ions is used as a color developing/subtracting agent, and which is capable of developing a color at a high temperature and removing color by heating at a lower temperature.
More specifically, the former is constituted of a matrix material formed of a thermoplastic resin or the like and an organic low-molecular material dispersed in the matrix material and has a property such as to be changed in state according to a temperature which is higher than a particular temperature T0 and at which it is maintained. That is, it has a recording layer having two state transition temperatures T1 and T2 (T1&lt;T2) higher than T0. If the recording layer is heated to and maintained at a temperature higher than T2 and is thereafter cooled to a temperature lower than T0, it becomes slightly opaque, i.e., comes into a maximum shading state. If the recording layer in this slightly-opaque state is heated to and maintained at a temperature equal to or higher than T1 and smaller than T2 and is thereafter cooled to a temperature lower than T0, it becomes transparent. These changes of state are mainly based on changes in the organic low-molecular material in the recording layer.
The latter recording medium can be changed into an organic compound by thermal energy control alone, i.e., by opening the lactone ring by high-temperature heating and can be returned to a colorless lueco compound by closing the lactone ring by low-temperature heating. This phenomenon is based on the structure of the color developing/subtracting agent and the reversibility of the lueco dyestuff, and can repeatedly be effected. A salt of gallic acid and fatty acid amine or the like is known as such a color developing/subtracting agent.
To increase the number of change repeating times of such reversible thermal recording mediums, a method of forming a transparent protective layer on the former type of medium (as disclosed in Japanese Patent Laid-Open Publication Nos. S57-82086, H2-131984, H2-81672 and H2-566) and a method of forming a thermoplastic resin protective layer on the latter type of medium have been practiced.
On the other hand, methods disclosed in U.S. Pat. No. 2,712,507, Japanese Patent Publication No.S51-35414, Japanese Patent Laid Open Nos.S58-211488, S59-229392, S60-214990 and H2-81679 are known as methods utilizing encapsulation, which is also utilized in accordance with the present invention. Almost all of these methods use a type of lueco dyestuff with which a color development reaction is started by breaking a capsule or a reaction is caused by permeation through a capsule wall, and are intended to improve the keeping quality of a thermosensible sheet.
With respect to use of a heating unit such as a thermal head, improvements in the conventional reversible thermal recording mediums are considered as only mitigation of the problem of transfer of a part of the recording layer to the heating unit or a change in the surface configuration of the recording layer in comparison with an arrangement in which an image is recorded and erased directly on a thermosensible medium (without a protective layer). That is, according to experiments made by the inventors of the present application, even if a protective layer formed of a thermoplastic resin or the like is used, the number of repeating times cannot be increased to 50 and there is the problem of a reduction in image quality due to transfer of the material of the protective layer or the recording layer to the heating unit (attachment of dust scraped off). This is because a recording mark is left by the heat and pressure of the thermal head, i.e., the heating unit so that the surface of the recording layer is toughened and the desired surface flatness is lost. Even if the heat resistance of the protective layer is improved by using a thermosetting type of UV setting type resin, the number of repeating times is at most 100, and a number of repeating times of 1,000 to 10,000, which is a limit of an essential physical or chemical change, cannot be obtained.